The present invention relates to a two-stage anodic oxidation process for aluminum which is particularly employed as a support material for offset-printing plates.
Support materials for offset-printing plates are provided, on one or both sides, with a radiation-sensitive (photosensitive) coating (reproduction coating), which is applied either directly by the user or by the manufacturer of precoated printing plates and with the aid of which a printing image of an original is produced by a photomechanical method. Following the production of a printing form of this type from the printing plate, the coating support comprises image areas which are ink-receptive in the subsequent printing process. Simultaneous with the image-production, a hydrophilic image-background for the lithographic printing operation is formed in the areas which are free from an image (non-image areas) in the subsequent printing process.
A coating support for reproduction coatings used in the manufacture of offset-printing plates must meet the following requirements:
Those portions of the radiation-sensitive coating, which are comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue and without any stronger attack on the support material by the developer. PA0 The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink. PA0 The radiation-sensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure. PA0 The support material should possess good mechanical stability, for example with respect to abrasion, and good chemical resistance, especially with respect to alkaline media. PA0 The direct current sulfuric acid process, in which anodic oxidation is carried out in an aqueous electrolyte which conventionally contains approximately 230 g of H.sub.2 SO.sub.4 per 1 liter of solution, for 10 to 60 minutes at 10.degree. to 22.degree. C., and at a current density of 0.5 to 2.5 A/dm.sup.2. In this process, the sulfuric acid concentration in the aqueous electrolyte solution can also be reduced to 8 to 10% by weight of H.sub.2 SO.sub.4 (about 100 g of H.sub.2 SO.sub.4 per liter), or it can also be increased to 30% by weight (365 g of H.sub.2 SO.sub.4 per liter), or more. PA0 The "hard-anodizing process" is carried out using an aqueous electrolyte, containing H.sub.2 SO.sub.4 in a concentration of 166 g of H.sub.2 SO.sub.4 per liter (or about 230 g of H.sub.2 SO.sub.4 per liter), at an operating temperature of 0.degree. to 5.degree. C., and at a current density of 2 to 3 A/dm.sup.2, for 30 to 200 minutes, at a voltage which rises from approximately 25 to 30 V at the beginning of the treatment, to approximately 40 to 100 V toward the end of the treatment.
As the base material for coating supports of this kind, aluminum is particularly frequently used, the surface of this aluminum being roughened, according to known methods, for example, by dry-brushing, slurry-brushing, sandblasting, or by chemical and/or electrochemical treatments. In order to increase the resistance to abrasion, electrochemically roughened substrates, especially, are additionally subjected to an anodizing step, in order to build up a thin oxide layer. These anodic oxidation processes are conventionally carried out in aqueous electrolytes which contain H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, H.sub.2 C.sub.2 O.sub.4, H.sub.3 BO.sub.3, amidosulfonic acid, sulfosuccinic acid, sulfosalicylic acid or mixtures thereof. The oxide layers built up in these aqueous electrolytes or electrolyte mixtures differ from one another in structure, layer thickness and resistance to chemicals. Roughened and anodically oxidized materials of this type also are of some importance in other technical fields, for example, in electrolytic capacitors or in the building industry. Aqueous solutions of H.sub.2 SO.sub.4 and/or H.sub.3 PO.sub.4 are particularly used in the commercial production of supports for offset-printing plates.
By way of example, the following standard methods are representative of the use of aqueous electrolytes containing H.sub.2 SO.sub.4 for the anodic oxidation of aluminum (see, in this regard, e.g., M. Schenk, Werkstoff Aluminium und seine anodische Oxydation (The Material Aluminum and its Anodic Oxidation), Francke Verlag, Bern, 1948, page 760; Praktische Galvanotechnik (Practical Electroplating), Eugen G. Leuze Verlag, Saulgau, 1970, pages 395 et seq., and pages 518/519; W. Huebner and C. T. Speiser, Die Praxis der anodischen Oxidation des Aluminiums (Practical Technology of the Anodic Oxidation of Aluminum), Aluminium Verlag, Duesseldorf, 1977, 3rd edition, pages 137 et seq.):
In the anodic oxidation of aluminum support materials for printing plates, described in European Pat. No. 0,004,569 (=U.S. Pat. No. 4,211,619), an aqueous electrolyte is used which contains from 25 to 100 g/l of H.sub.2 SO.sub.4 and the Al.sup.3+ ion content of which is adjusted to values exceeding 10 g/l.
Aluminum oxide layers produced by these methods are amorphous and, in the case of offset-printing plates, conventionally have a layer weight of about 0.5 to 10 g/m.sup.2, corresponding to a layer thickness of about 0.15 to 3.0/.mu.m. When a support material which has been anodically oxidized in this way is used for offset-printing plates, it has the disadvantage that the oxide layers produced in H.sub.2 SO.sub.4 electrolytes have a comparatively low resistance to alkaline solutions, such as are used to an increasing extent, for example, in the processing of pre-sensitized offset-printing plates, and preferably in up-to-date developing solutions for radiated negative-working or, in particular, positive-working radiation-sensitive coatings.
The anodic oxidation of aluminum in aqueous electrolytes containing phosphoric acid is also known, as discussed below.
German Auslegeschrift No. 1,671,614 (=U.S. Pat. No. 3,511,661) discloses a process for manufacturing a lithographic printing plate in which the aluminum support is anodically oxidized in an at least 10% strength aqueous solution of H.sub.3 PO.sub.4, at a temperature of at least 17.degree. C., until the layer of aluminum oxide has a thickness of at least 50 nm.
German Offenlegungsschrift No. 1,809,248 (=U.S. Pat. No. 3,594,289) discloses a process, in which an aluminum support material for printing plates is anodically oxidized for 2 to 10 minutes, in a 5 to 50% strength aqueous solution of H.sub.3 PO.sub.4, at a current density of 0.5 to 2.0 A/dm.sup.2 and a temperature of 15 to 40.degree. C.
The anodic oxidation of aluminum support materials for printing plates, which is described in German Offenlegungsschrift No. 2,507,386 (=British Pat. No. 1,495,861) is carried out in a 1 to 20% strength aqueous solution of H.sub.3 PO.sub.4 or of polyphosphoric acid at 10 to 40.degree. C., using an alternating current at a current density of 1 to 5 A/dm.sup.2 (1 to 50 V).
Although an oxide layer produced in phosphoric acid is frequently more stable with respect to alkaline media than an oxide layer which has been produced in an electrolyte based on a H.sub.2 SO.sub.4 solution, and additionally exhibits a number of other advantages, such as lighter surface, better water/ink balance or low adsorption of dyes ("staining" in the non-image areas), it nevertheless also possesses significant disadvantages. The oxide-layer weights which can be produced in a modern strip-processing unit for the manufacture of printing-plate supports, using voltages and dwell times which are technically appropriate, range, for example, up to only approximately 1.5 g/m.sup.2, a layer thickness which naturally offers less protection against mechanical abrasion than a thicker oxide layer, produced in a H.sub.2 SO.sub.4 electrolyte. Due to the larger pore volume and pore diameter in an oxide layer which has been produced in H.sub.3 PO.sub.4, the mechanical stability of the oxide itself is also lower, which results in further losses with regard to abrasion resistance.
Also, processes have already been disclosed which attempt to combine the advantages of the two electrolytes, by using electrolyte mixtures composed of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4 or employing a two-stage treatment procedure.
In the process for manufacturing aluminum support materials for printing plates, according to German Offenlegungsschrift No. 2,251,710 (=British Pat. No. 1,410,768), aluminum is first anodically oxidized in an electrolyte containing H.sub.2 SO.sub.4, to form an oxide layer which is then post-treated in a 5 to 50% strength aqueous solution of H.sub.3 PO.sub.4, without the action of an electrical current. The actual oxide layer is stated to possess a weight per unit area of 1 to 6 g/m.sup.2, but a significant decrease of this weight, for example, by about 2 to 3 g/m.sup.2 per minute of immersion time, occurs upon immersion in the aqueous H.sub.3 PO.sub.4 solution. It is also stated that it is possible to perform an electrochemical treatment in the H.sub.3 PO.sub.4 solution (Example 11) or to employ a mixed electrolyte composed of H.sub.3 PO.sub.4 and H.sub.2 SO.sub.4 (Example 12), the oxide layer being likewise reduced in these cases.
U.S. Pat. No. 3,940,321 also describes a two-stage anodic oxidation, first in an electrolyte based on H.sub.2 SO.sub.4, and then in an electrolyte based on H.sub.3 PO.sub.4, using a direct current at a voltage of 10 to 15 V (1 to 15 A/dm.sup.2 current density) in both stages. The aqueous electrolytes which are employed contain, in the first stage, from 5 to 50% of acid and, in the second stage, from 20 to 60% of acid.
A mixed electrolyte composed of H.sub.2 SO.sub.4 and H.sub.3 PO.sub.4, which is used in the production of support materials for printing plates, is described in European Pat. No. 0,008,440 (=U.S. Pat. No. 4,229,226), in which a specific content of aluminum ions is additionally mentioned.
In European Pat. Nos. 0,007,233 and 0,007,234, aluminum support materials for printing plates are anodically oxidized by passing them, as center conductors, first through a bath containing a 45% strength aqueous H.sub.3 PO.sub.4 solution and an anode and then into a bath containing a 15% strength aqueous H.sub.2 SO.sub.4 solution and a cathode. The two electrodes can also be connected to a source of alternating voltage (in each case about 16 to 21 V, 2 A/dm.sup.2). In the treatment with direct current, the first bath substantially serves for producing the electrical contact. In the treatment with alternating current, the respective half-wave, which results in the aluminum being made the anode, can effect an anodic oxidation already in the first bath.
British Patent Application No. 2,088,901 discloses a two-stage anodic oxidation process for aluminum support materials for printing plates, which uses, in the first stage, an aqueous electrolyte containing 250 to 400 g of H.sub.3 PO.sub.4 per liter, for 15 to 240 seconds, at a voltage from 15 to 35 V and at a temperature from 15.degree. to 46.degree. C. and, in the second stage, an aqueous electrolyte containing 20 to 150 g of H.sub.2 SO.sub.4 and 250 to 380 g of H.sub.3 PO.sub.4 per liter, under the above-specified conditions. In particular, the voltage employed in the second stage should be higher than or equal to the voltage employed in the first stage; the voltage applied in the examples is invariably based on a direct-current source.
The processes with mixed electrolytes may effect (with increasing H.sub.3 PO.sub.4 content) an approximation of the properties of the oxide layer to the properties obtained in an anodic oxidation in pure aqueous H.sub.3 PO.sub.4 solutions, but they do not reach these properties. On the other hand, the positive properties of an anodic oxidation in pure aqueous H.sub.2 SO.sub.4 solutions, e.g., thickness of oxide layer, abrasion-resistance, also decline. Moreover, a bath monitoring procedure (in the case of a solution containing several components) is very expensive in terms of production technology, and is difficult to control. The two-stage anodic oxidation or treatment method, leads to a situation wherein the oxide layer which has been built up in the H.sub.2 SO.sub.4 electrolyte is redissolved in the H.sub.3 PO.sub.4 solution to an excessive extent, under the conditions hitherto known. This is also the case with the prior art processes, in which this sequence of stages is reversed, particularly if an alternating current is used and due to the very high concentrations of H.sub.3 PO.sub.4 in the electrolyte. In the process variant which employs an acid mixture composed of H.sub.3 PO.sub.4 and H.sub.2 SO.sub.4 in the second stage, problems with bath-monitoring are again encountered. Moreover, the process variant using a single circuit for the two stages can be disadvantageous, since it is more difficult to control from the point of view of production engineering.